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Coherence-guided InSAR deformation analysis in the presence of ongoing land surface changes in the Imperial Valley, California

Junle Jiang, & Rowena B. Lohman

Published November 25, 2020, SCEC Contribution #10073

While the quality and availability of Interferometric Synthetic Aperture Radar (InSAR) data has dramatically improved in recent years, InSAR analysis and interpretation remain challenging in actively deforming regions with extensive agricultural activities, where vegetation changes and soil moisture variability can degrade data quality or introduce confounding signals. Here we use Sentinel-1 satellite imagery for the Imperial Valley, California, over the period of 2015–2019 to explore how factors specific to land surface changes may impact InSAR time series and to resolve time-varying deformation due to tectonic processes and geothermal energy production. We examine the temporal variability of data quality, via interferometric phase coherence, at high spatial resolution, taking into account the observation that some agricultural fields lie fallow for long time intervals punctuated by periods of cultivation. This strategy allows us to better distinguish signals and noise associated with agricultural activities, shoreline changes, or surface soil conditions. A series of masking, interpolation, and filtering steps facilitate phase unwrapping, and the unwrapped, unfiltered product is then recovered, reducing artifacts from spatial filtering. We adopt model-based tropospheric corrections to improve time series results, particularly in regions with high topographic relief, along with the use of distributed reference points to render a more uniform error structure. We validate InSAR observations with continuous GPS where available and find that the estimates of average line-of-sight (LOS) velocity over the valley from InSAR and GPS agree to ~3 mm/yr in areas with good data coverage. Discrepancies between the two estimates often exist in areas with lower InSAR data quality; in better-constrained areas, they appear to reveal signals attributable to surficial processes occurring in the uppermost soil layers. We observe a diverse suite of natural signals over multiple spatial scales, including steady interseismic deformation, seasonal lake-level-modulated signals at the southeastern Salton Sea shore, and transient slow slip on the Superstition Hills fault. In addition, we observe complex deformation at four geothermal fields within the valley. Extensive subsidence at the Salton Sea geothermal field suggests spatial overlap of anthropogenic and tectonic deformation, interspersed with potential surficial signals. Geothermal sites at East Mesa, North Brawley, and Heber exhibit smaller-amplitude, more localized deformation, often with nonlinear temporal trends. Our analysis demonstrates the need to assess whether InSAR signals result from surficial changes or deeper sources, and produces robust ground deformation data in support of efforts to study subsurface processes, manage geothermal operations, and improve hazard assessments.

Key Words
InSAR, GPS, Geodetic imaging, Ground deformation, Uncertainty quantification, Regional tectonics, Anthropogenic activity, Land use, Tropospheric noise, Soil moisture, Lake level change, Fault creep, Geothermal operation

Jiang, J., & Lohman, R. B. (2020). Coherence-guided InSAR deformation analysis in the presence of ongoing land surface changes in the Imperial Valley, California. Remote Sensing of Environment,. doi: 10.1016/j.rse.2020.112160.

Related Projects & Working Groups
SDOM, Tectonic Geodesy, Earthquake Geology